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P bus and I bus

P bus and I bus

Cars with petrol engines


Cars with diesel engines


A bus is understood to be the leads over which information is sent digitally and serially. Digital means that the voltage difference between the leads has only two values, roughly 0 V and 5 V. The information is coded so that different combinations of 0 V and 5 V pulses have different meanings.

Serial means that the information is sent in ”packets” which are transmitted one after the other in rapid succession.

In Saab 9-5 all of the car's control modules except ABS, DAH and AHL are connected to a bus. On cars with Traction Control or ESP, the TC/ABS or ESP control module is connected to the P-bus.

The bus consists of a P-bus (Powertrain Bus) and an I-bus (Instrument Bus). Both buses are connected to the MIU (Main Instrument Unit). The buses are electrically isolated from each other.

The diagnostic tool is not connected directly to the bus but communicates via the DICE, one of the control modules connected to the I-bus, and so has access to all control modules connected to the bus.

The data transfer rate of the P-bus is ten times faster than that of the I-bus. The reason for this is that the powertrain systems need information with the least possible delay, as for example when providing air mass compensation when the selector lever is moved from N to D or torque limitation in connection with gear changing.

All the information sent from one control module is accessible for all other control modules on the bus. The MIU is responsible for ensuring that information available on one bus is also available on the other bus.

Vehicle speed is important information for many control modules. Because ABS is not connected to a bus, the vehicle speed signal travels on a lead from ABS to MIU. MIU then sends the information on vehicle speed over the busses. On cars with Traction Control or ESP, MIU receives speed information from the TC/ABS or ESP control module via the P-bus.

The control modules send out information on the bus at regular intervals. The time between two transmissions depends on the information being sent and varies between 10 milliseconds (0.010 seconds) and 1 second. Information is also sent out by the control modules whenever the information changes.

Information is transferred between control modules over two leads, bus+ (green cable) and bus- (white cable). Both leads of the bus are twisted to reduce susceptibility to electrical interference.